1. Field of the Invention
The present invention relates to a casting method of a metal material for obtaining a molding product, for example, of an aluminum alloy by a high pressure die casting, and particularly relates to a technique of shortening a cycle time of casting.
2. Related Art
In a die casting such as a high pressure die casting, a member referred to as a distributor is sometimes disposed at a molten metal introduction portion of a casting die for introducing a molten metal to a cavity of the die. In this case, the molten metal injected under pressure from a plunger during casting abuts against the distributor and the molten metal is introduced through a runner formed to the distributor into the cavity of the die. Since the distributor is a member in components of the die which is to be in contact with the molten metal at first, the distributor repetitively receives cooling and heating cycles. In addition, since the distributor is to be in contact with a biscuit portion having a largest thickness in the casting product or a runner, a considerable cooling efficiency is required. Moreover, it is required to smoothly flow the molten metal to the cavity and to solidify the molten metal, a cooling means is provided in the distributor (for example, JP-A-2006-239738).
In the configuration of JP-A-2006-239738, the cooling efficiency of the distributor is improved by a structure of a cooling circuit. By the way, it is also considered to use a metal material having a high thermal conductivity as a material of the distributor, in order to enhance the cooling efficiency. Copper alloys can be considered as the metal material having the high thermal conductivity. However, since the copper alloys are disadvantageous with a view point of wear resistance and strength, alloy tool steels such as SKD (Steel Kogu Die) have been used as the material of the distributor. Further, in a case of using the copper alloys, since copper tends to react with aluminum and suffers from melting loss, it is necessary to form a surface treating film on the surface of the copper by applying a surface treatment (Cr—N system, DLC, Ti—N system, Ti—Al—N system, etc.) by PVD, CVD, PCVD, etc.
However, since the surface treating film has a heat insulating property, the thermal conductivity of the distributor is lowered by the surface treating film. Further, since the surface treating film of the type mentioned above has low oxidation resistance, it also involves a problem of tending to cause cracking or defoliation. When cracking occurs to the surface treating film, since melting loss occurs in the distributor, it is necessary to previously detach the distributor from the die and apply a surface treatment but this makes the process complicate. Further, even in a distributor made of copper alloys, internal cooling water boils when the cycle time is shortened to lower the cooling efficiency and the temperature of the material (molten material) is increased to result in various disadvantages in the operation (remarkable disadvantage in the operation efficiency) such as bursting, scorching, or caulking in a biscuit portion.